Predation detection animal tracking tag

ABSTRACT

A magnetic tracking tag for animals may detect the occurrence of a predation event. The tag may include a pH sensitive material that degrades in the environment of a predator&#39;s gut. The degradation of the pH sensitive material causes a change in a detectable magnetic field of the tag, which allows the tag to detect the predation event and adjust its operation accordingly.

FIELD OF THE INVENTION

The current description relates to a tracking tag for animals, and inparticular to a tracking tag capable of detecting a predation event.

BACKGROUND

Fish or other marine animals may be tagged with tracking devices inorder to track their movement. The tracking tags transmit ultrasonicpulses that can be detected at one or more receivers deployed at variouslocations in a body of water under study.

The tags used for tracking marine animals are often implanted within thecoelomic cavity of the animal being tracked. Typically, tracking tagsperiodically transmit ultrasonic pulses to communicate a uniqueidentifier of the tag that allows individual animals to be tracked.Additionally, the tracking tags may also communicate other datacollected by the tag, such as temperature and acceleration information.Once the tracking tags are activated, they typically will continue totransmit until a battery dies.

When a tagged marine animal is eaten by a predator, the tracking tagoften will continue to operate within the predator's body. Researchershave been unable to determine if the tracking tag was operating in atagged animal or within a predator.

An additional, alternative and/or improved tracking tag for marineanimals is desirable.

SUMMARY

In accordance with the present disclosure there is provided a tag fortracking an animal comprising: a first magnet at least partially encasedin a pH sensitive material exposed to an external environment of thetag, the pH sensitive material degrading in the presence of an acidicenvironment; a sensor for detecting a magnetic field; a magneticattractor providing a force to induce a change in the magnetic fielddetectable by the sensor upon release of the first magnet as a result ofat least partial degradation of the pH sensitive material; and circuitryfor providing a predation event trigger based upon the change in themagnetic field detectable by the sensor.

In a further embodiment of the tag, the magnetic attractor comprises asecond magnet.

In a further embodiment of the tag, the second magnet is permanentlyaffixed to the tag.

In a further embodiment of the tag, the second magnet causes the firstmagnet to move relative to the sensor when the pH sensitive material issufficiently degraded to release the first magnet.

In a further embodiment of the tag, the second magnet is arranged withits magnetic field perpendicular to the magnetic field of the firstmagnet when the first magnet is at least partially encased in a pHsensitive material, and the second magnet causes the first magnet torotate when the pH sensitive material is sufficiently degraded torelease the first magnet.

In a further embodiment of the tag, the sensor is capable of detectingan orientation of a magnetic field, a change in the orientation of themagnetic field, a magnitude of the magnetic field, a change in themagnitude of the magnetic field or a combination thereof.

In a further embodiment of the tag, the second magnet is arranged withits magnetic field perpendicular to a particular axis of the sensor.

In a further embodiment of the tag, the sensor is sensitive to themagnetic field of the first magnet when it is oriented parallel with theparticular axis of the sensor, and insensitive to the magnetic field ofthe first magnet that is oriented perpendicular to the particular axisof the sensor.

In a further embodiment of the tag, the second magnet attracts the firstmagnet out of a detection range of the sensor when the pH sensitivematerial is sufficiently degraded to release the first magnet.

In a further embodiment of the tag, the first magnet and the secondmagnet are secured or located against or adjacent to each other by thepH sensitive material with their respective magnetic fields aligned suchthat their respective magnetic fields add together constructively,wherein the first magnet and the second magnet tend to rotate or moverelative to each other when released from the pH sensitive material suchthat their respective magnetic fields add destructively when moved orrotated.

In a further embodiment of the tag, the sensor is capable of detecting astrength of the magnetic fields or a change in the strength of themagnetic fields.

In a further embodiment of the tag, the magnetic attractor comprises anun-magnetized ferromagnetic material permanently affixed to the tag.

In a further embodiment of the tag, the magnetic attractor attracts thefirst magnet out of a detection range of the sensor when the pHsensitive material is sufficiently degraded to release the first magnet.

In a further embodiment of the tag, the first magnet comprises adegradable magnet.

In a further embodiment of the tag, the degradable magnet comprisesparticles of ferromagnetic material bonded together by the pH sensitivematerial, the particles of the ferromagnetic material being magnetizedsubsequent to bonding together to impart a net magnetic field to thedegradable magnet.

In a further embodiment of the tag, the degradable magnet releases themagnetized particles of magnetic material when the pH sensitive materialis degraded resulting in a reduced net magnetic field.

In a further embodiment of the tag, the acidic environment is a gut of apredator animal.

In a further embodiment of the tag, the pH sensitive material does notdegrade substantially in a neutral or basic environment.

In a further embodiment of the tag, the neutral or basic environment isa coelomic cavity of the animal.

In a further embodiment of the tag, the first magnet is affixed to thetag by using the pH sensitive material as an adhesive to affix the firstmagnet to the tag.

In a further embodiment of the tag, the first magnet is at leastpartially encased in the pH sensitive material to form a plug that isaffixed to the tag using an adhesive.

In a further embodiment of the tag, the first magnet is at leastpartially encased in the pH sensitive material to form a plug that ismechanically retained by at least a portion of a body of the tag.

In a further embodiment of the tag, the pH sensitive material comprisesa chitosan.

In a further embodiment of the tag, the pH sensitive material is castfrom a slurry of the chitosan and a solvent.

In a further embodiment of the tag, the solvent is selected from:L-ascorbic acid; citric acid; acetic acid; and hydrochloric acid.

In a further embodiment of the tag, the solvent is citric acid.

In a further embodiment of the tag, the solvent is acetic acid.

In a further embodiment of the tag, the pH sensitive material comprisesa film having a thickness of at least 0.05 mm.

In a further embodiment of the tag, the pH sensitive material comprisesa film having a thickness of at least 0.20 mm.

In a further embodiment of the tag, the pH sensitive material comprisesa plasticizing agent.

In a further embodiment of the tag, the plasticizing agent is selectedfrom: glycerol; ethylene glycol; poly ethylene glycol; erythritol; oleicacid; propylene glycol; di-hydroxyl stearic acid; and sorbitol.

In a further embodiment of the tag, the plasticizing agent is glycerol.

In a further embodiment of the tag, the pH sensitive material is treatedwith a cross-linking agent.

In a further embodiment of the tag, the cross-linking agent is selectedfrom: sodium citrate; sodium sulfate; and calcium chloride.

In a further embodiment of the tag, the pH sensitive material comprisesa filler material to control shrinkage of the pH sensitive material.

In a further embodiment of the tag, the filler material comprisesmicrospheres.

In a further embodiment of the tag, the microspheres comprise glassmicrospheres.

In a further embodiment of the tag, the filler material is present in anamount of between 50% and 150% by volume of the pH sensitive materialprior to drying.

In a further embodiment of the tag, the amount of filler materialpresent is between 83% and 111% by volume of the pH sensitive materialprior to drying.

In a further embodiment, the tag comprises a microprocessor forcontrolling one or more functions of the tag based upon the predationevent trigger.

In a further embodiment of the tag, the circuitry for providing thepredation event trigger is provided by the microprocessor.

In a further embodiment of the tag, the microprocessor operates in atleast one of a first mode or a second mode based on the predation eventtrigger.

In a further embodiment of the tag, the microprocessor switches fromoperating in the first mode to operating in the second mode based on thepredation event trigger.

In a further embodiment of the tag, the microprocessor logs informationrelated to the predation event trigger.

In a further embodiment of the tag, the microprocessor logs informationto non-volatile memory of the tag.

In a further embodiment of the tag, the microprocessor maintains ameasure of elapsed time since the predation event trigger, and whereinthe logged information comprises the elapsed time since the predationevent trigger event.

In a further embodiment, the tag comprises a transmitter fortransmitting information related to the predation event trigger.

In a further embodiment of the tag, the transmitter comprises anacoustic transducer.

In a further embodiment of the tag, the transmitter comprises a radiofrequency (RF) transmitter.

In a further embodiment of the tag, the RF transmitter comprises anactive RF transmitter.

In a further embodiment of the tag, the RF transmitter comprises apassive RF transmitter.

In a further embodiment of the tag, the passive RF transmitter comprisesan RFID transmitter.

In a further embodiment of the tag, the microprocessor maintains ameasure of elapsed time since the predation event trigger, and whereinthe transmitted information comprises the elapsed time since thepredation event trigger event.

In a further embodiment of the tag, the measure of elapsed timetransmitted by the transmitter is encoded in non-linear fashion.

In a further embodiment of the tag, the microprocessor operates in atleast a configuration mode for transferring data to the tag to configureoperation of the microprocessor.

In a further embodiment of the tag, a varying magnetic field is used fortransferring data to the tag when the microprocessor is in theconfiguration mode.

In a further embodiment of the tag, the microprocessor further operatesin at least a calibration mode for determining a value for acompensation magnetic field to allow detecting of the varying magneticfield used for transferring data in the presence of a constant magneticfield of at least the first magnet.

In a further embodiment of the tag, the microprocessor calculates thevalue for the compensation magnetic field and transmits the calculatedvalue for the compensation magnetic field to an activation device.

In a further embodiment of the tag, the tag transmits an indication of adetected magnetic field in order to allow an activation device tocalculate the value for the compensation magnetic field.

In a further embodiment of the tag, the animal is an aquatic animal.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects and advantages of the present disclosure will becomebetter understood with regard to the following description andaccompanying drawings in which:

FIG. 1 depicts an environment in which tracking tags may be used;

FIGS. 2A, 2B, 2C depict a predation tracking tag;

FIGS. 3A, 3B depict a further predation tracking tag;

FIGS. 4A, 4B depict a further predation tracking tag;

FIGS. 5A, 5B depict a further predation tracking tag;

FIG. 6A, 6B depict a further predation tracking tag;

FIG. 7 depicts a further predation tracking tag;

FIGS. 8A, 8B, 8C, 8D, 8E, 8F depict further predation tracking tags;

FIGS. 9A, 9B, 9C, 9D depict a further predation tracking tag;

FIGS. 10A, 10B, 10C, 10D depict a further predation tracking tag;

FIGS. 11A, 11B depict a further predation tracking tag;

FIG. 12 depicts an activator for activating a predation tracking tag;

FIG. 13 depicts components of a predation tracking tag and an activator;

FIG. 14 depicts components of a further predation tracking tag and anactivator;

FIG. 15 depicts components of the predation tracking tag of FIGS. 8A-F,9A-D, 10A-D and/or 11A-B, and an activator;

FIG. 16 depicts a method of activating a tracking tag;

FIG. 17 depicts signals associated with activating a tag;

FIG. 18 depicts further signals associated with activating a tag;

FIG. 19 depicts a method of operating a tracking tag;

FIG. 20 shows a graph of the effect of film thickness on swelling anddegradation of the films;

FIG. 21 shows a graph of the effect of plasticizers on swelling of thefilms;

FIG. 22 shows a graph of the effect of concentration of glycerol tochitosan on swelling;

FIG. 23 shows a graph of the effect of cross-linkers on the swelling anddegradation of films;

FIG. 24 shows a graph of the effect of sodium citrate concentration onswelling and degradation;

FIG. 25 shows a graph of the effect of cross-linking time on filmswelling and degradation;

FIG. 26 shows a graph of the effect of additives on the long-termstability of the films; and

FIG. 27 shows a graph of the effect of accelerated aging on the films.

DETAILED DESCRIPTION

A tracking tag for tracking animals is described further herein. Thetracking tag is capable of detecting if the animal being tracked hasbeen eaten by a predator and adjusting its operation upon detection ofthe predation event. The tracking tag uses a magnet secured in place bya pH sensitive material that degrades in the presence of the acidicenvironment of a predator's gut. Once sufficiently degraded, the magnetmoves and the resulting magnetic field, or the change in the magneticfield may be used as an indication that a predation event has occurred.The tag may include a component, such as an additional magnet orun-magnetized ferromagnetic material, that provides a biasing force tothe magnet so that once the magnet is released from the degraded pHsensitive material the magnet will tend to move causing a detectablechange in the magnetic field.

FIG. 1 depicts an environment 100 in which tracking tags may be used.Although FIG. 1 depicts a predation tracking tag being used within anaquatic environment, the predation tracking tag may be used in othernon-aquatic environments. A fish 102 is tagged with a tracking device104, referred to as a tracking tag, or simply a tag. Tracking tags maybe implanted within the coelomic cavity of the animal. Additionally oralternatively, the tracking tag may be externally attached to theanimal, for example by adhering or affixing to an animal's coat or fin.The tag 104 periodically emits ultrasonic pulses that are received byone or more receivers 106 that are located within the body of water orarea being studied. The receivers may be anchored 108 in place or may besuspended by a buoy 110, or any other suitable means of positioning thereceiver at a desired location. The receiver 106 detects the ultrasonicpulses transmitted by tags. The detection events are recorded and storedby the receiver 106. The recorded events can be retrieved from each ofthe receivers. For example, the data can be periodically downloaded to acomputer such as a laptop 112. Although depicted as being carried outwhile the receiver 106 remains in position, the data may also beretrieved from the receiver 106 by a physical connection to thereceiver, which may require the receiver 106 to be retrieved. The lengthof time the receiver remains in place may vary depending upon therequirements of the study. Further, it is possible to track a tag inreal time; however, this is typically performed by having a receiverlocated on a boat and following the marine animal being tracked as itmoves about.

The tags may communicate data using various techniques. For example, thetags may continuously transmit signals, or may periodically transmitsignals. The time interval at which the tags transmit signals may varyand may be adjustable. For example, a tag may transmit signals every 1second, 5 seconds, 10 seconds, 15 seconds, 30 seconds 60 seconds, or atother intervals. In addition to varying when tags transmit information,the information that is transmitted may also be varied. For example, thetag may simply transmit a unique, or unique within a particular set oftags, identifier (ID). Receivers may detect the periodic transmission ofthe unique IDs and record the detection events, along with the time ofthe detection. The movement of the animal being tracked can bereconstructed from the various detection events with correspondingunique IDs. In addition to the unique ID, the tag may also transmitother information such as a sequence number that increases with eachtransmission, or other data that is tracked by the tag depending uponavailable sensors in the tag. For example, the tag may also indicatereadings from a temperature sensor, acceleration sensors or other typesof sensors as may be provided by the tag.

Tags may last for a varying period of time depending upon the operatingcharacteristics of the tag as well as the power supply of the tag.Generally, a tag is activated prior to being attached to or implanted inthe animal being tracked. Once the tag is activated it will typicallycontinue to operate until it does not have sufficient power. If a marineanimal being tracked, such as fish 102, is eaten by a predator such asfish 114, the tag may continue to operate and the movement of thepredator may be incorrectly associated with the marine animal that waseaten.

Tags as described herein can detect a predation event and adjust theiroperation accordingly. When a tag detects that the marine animal hasbeen eaten, the tag may stop transmitting further information, or maycontinue transmitting information but provide an indication that apredation event was detected, for example by transmitting a secondary IDof the tag. The secondary ID of the tag may include the original uniqueID or be otherwise associated with the original ID so that the animalthat was eaten can be determined. For example the first unique ID fortracking a tagged living animal may be 123A while the second unique IDof the tag used when a predation event has been detected may be 123B. Inthis example of the IDs it is assumed that the 123 portion of both IDsis unique with regard to other tags. The tag does not detect thepredation event directly, but rather detects a change associated withthe animal being eaten. The tag may use a pH sensitive material thatdegrades in the acidic environment of the gut of a predator. Thedegradation of the pH sensitive material can be detected by the tag and,as such, the tag can adjust its operation accordingly.

In addition to providing an indication that a predation event hasoccurred, for example by transmitting a different unique ID when apredation event occurs, the tag may transmit additional informationassociated with the predation event. For example, the tag may transmit ameasure of elapsed time since the detection of a change in thecharacteristic of the tag, which change is associated with the predationevent. The measure of elapsed time may be transmitted in the ultrasonicsignal. The measure may increase with the passage of time through theoperating life of the tag. The elapsed time measure may be encoded in alinear or non-linear fashion or format. Encoding the elapsed time in anon-linear fashion may provide better resolution during early stagesfollowing predation compared to late stages. For example, the measuremay change in 15 minute increments in the short term after the predationevent, but use one day increments late in its operating life, andpossibly using various other increments in the interim as time passes.The time since the detection of a change in the characteristic of thetag may be tracked by the microcontroller or other appropriatecomponents of the tag.

Although the tag 104 is described as communicating information,including information related to a predation event, to a receiver usingultrasonic pulses, it is possible for the predation tracking tag tocontrol the tag operation in other ways in response to a detectedpredation event. For example, the tag could transmit the informationusing additional or alternative transmitters, such as radio frequency(RF) transmitters. Acoustic transmitters may be well suited for aquaticenvironments, however, it is possible for RF transmitters to be used inaquatic environments, although water characteristics, including forexample salt content, may reduce a useful transmission range of the RFtransmitter. Additionally, or alternatively, the tag could log, orstore, the occurrence of the predation event in a tag memory for laterretrieval upon retrieval of the predation tracking tag. It will beapparent that the predation tracking tags may perform differentfunctions or operate in different modes when a predation event occurs.

FIGS. 2A, 2B, 2C depict a predation tracking tag. The predation trackingtag 200 may be used to track a tagged animal. The tag 200 detects apredation event, i.e., the tagged animal being eaten by a predator. Thetag 200 comprises a main body 202 that houses the main tag components.The tag 200 may vary in size from a few millimeters to a few centimetersor more depending upon the size of the animal being tracked. The tag 200includes a sensor 204 that detects an electrical characteristic of thetag 200. The sensor 200 comprises a substrate 206 that provides supportfor two electrodes 208 a, or 208 b. The impedance or other electricalcharacteristics of the electrodes can be detected by a microcontrollerof the tag.

A coating or film of a pH sensitive material 210 a may be formed overthe electrodes 208 a, 208 b as depicted in FIG. 2B. The pH sensitivematerial is selected to degrade, preferably quickly, in the acidicenvironment of a predator's gut, while resisting degradation in aneutral or basic environment, such as a coelomic cavity of an animal inwhich the tag has been implanted. As such, when the tag remains in thecoelomic cavity of the animal being tracked, the pH sensitive materialstays intact covering the electrodes 208 a, 208 b and as such a firstvalue will be measured for the particular electrical characteristicbeing monitored, such as impedance or resistance.

If the tagged animal is eaten, the animal will be digested within thegut of the predator, and as such the tag 200 will be exposed to theacidic environment of the predator's gut. The acidic environment willdegrade the pH sensitive material 210 b covering the electrodes asdepicted in FIG. 2C. As the pH sensitive material is degraded, theelectrodes 208 a, 208 b will be exposed to the environment and thepreviously measured value of the electrical characteristic will change.The different value is detected and used by the tag to determine that apredation event has occurred. Once the tag, or more particularly amicrocontroller of the tag, has determined that the predation event hasoccurred, the operation can be adjusted accordingly, for example bystopping further transmission, or by altering the transmitted ID of thetag used to indicate that the tag has detected a predation event.Additional, or alternative, actions such as logging information mayoccur upon detection of the predation event.

Various pH sensitive materials may be used, and a selection of possiblepH sensitive materials are described further herein. The pH sensitivematerial should resist degradation when in the environment associatedwith a living animal being tracked. The pH sensitive material shouldresist degradation in such an environment for a relatively long periodof time, such as the expected operating lifetime of the tag. Althoughthe pH sensitive material should resist degradation within a normaloperating environment, such as within a coelomic cavity, it shoulddegrade, preferably quickly, within the acidic environment. For example,the pH sensitive material may degrade quickly enough to expose theelectrodes within 60 to 120 minutes of a tagged animal being eaten by apredator. The composition of the pH sensitive material, as well as anamount of the pH sensitive material used to cover the electrodes, and sowhich must degrade to expose the electrodes, can be adjusted to meet therequired characteristics. Further, the composition may include one ormore additives that may affect the detection of the characteristic and,as such, facilitate the determination of whether or not the pH sensitivematerial is still intact or has substantially degraded. The pH sensitivematerial may be considered to have substantially degraded once it hasdegraded enough that a change in the characteristic can be detected inorder to indicate the occurrence of the predation event.

FIGS. 3A, 3B depict further predation tracking tags. The tags depictedare similar to the tag 200 described above; however, the geometry of theelectrodes differ. The predation tracking tag 300 comprises a body 302and shortened and rounded predation sensor 304 coupled to theelectronics 312 of the predation tracking tag 302. The predation sensor304 comprises electrodes 308 a, 308 b on a substrate 306. The electrodes308 a, 308 b are covered with a pH sensitive material 310 that degradesin the acidic environment of a predator's digestive tract. When ananimal tagged with the predation tracking tag 302 is eaten by apredator, the pH sensitive material 310 dissolves in the digestive tractof the animal. Once the pH sensitive material 310 degrades, theelectrodes 308 a, 308 b are exposed to the environment and theelectronics 312 may detect a change in the electrical characteristics ofthe electrodes. The shortened and rounded geometry of the predationsensor 304 may be appropriate for insertion into the coelomic cavity dueto its shortened length and lack of corners.

FIGS. 4A, 4B depict a further predation tracking tag. The tag 400 issimilar to the tags described above and comprises a body 402 and anexternal sensor component capable of measuring a characteristic of thetag. The sensor 404 comprises two resilient electrodes, each comprisinga resilient material 406 a, 406 b and a conductive material 408 a, 408b. As depicted in FIG. 4A, the pH sensitive material 410 a is cast inorder to prevent the resilient electrodes from contacting each other.When an acidic environment substantially degrades the pH sensitivematerial 410 b as depicted in FIG. 4B, the resilient material 406 a, 406b of the electrodes cause the electrically conductive material 408 a,408 b to come into contact with each other. The tag 400, or moreparticularly a microcontroller of the tag, can detect the newlyestablished electrical connection between the two electrodes, which maybe used as an indication that the marine animal being tracked was eatenby a predator. Once the tag detects the electrical connection betweenthe electrodes, the tag may switch from the first mode of operationassociated with tracking the animal originally tagged to a second modeof operation associated with the originally tagged animal being eaten bya predator.

FIGS. 5A, 5B depict a further predation tracking tag. The tag 500 issimilar to the tags described above and can detect a change in acharacteristic of the tag 500 when the environment the tag is in changesfrom a neutral or basic environment, such as that found in coelomicfluid, to an acidic environment such as that found in the gut of apredator. The tag 500 comprises a body 502 and a sensor for measuring acharacteristic of the tag. The sensor is depicted as a strain gauge 504a or other similar sensor that changes its electrical characteristicsbased on the shape of the sensor 504 a. As depicted in FIG. 5A, thestrain gauge 504 a may be cast within a pH sensitive material 506 a inorder to maintain the strain gauge 504 a in a first configuration. Asdepicted in FIG. 5B, when the acidic environment substantially degradesthe pH sensitive material 506 b, the strain gauge 504 b changesconfigurations, reducing the strain, which may be detected by the tagand used as an indication that the animal being tracked was eaten by apredator.

FIG. 6A, 6B depict a further predation tracking tag. The tag 600 issimilar to the tags described above and can detect a change in acharacteristic of the tag 600 when the environment the tag is in changesfrom a neutral or basic environment, such as that found in coelomicfluid, to an acidic environment such as that found in the gut of apredator. The tag 600 comprises a body 602 and a sensor for measuring acharacteristic of the tag. The sensor is depicted as an infrared (IR)transmitter and receiver pair 608 a, 608 b. The IR transmitter 608 a andIR receiver 608 b are encased in pH sensitive material 610 a thatdegrades in an acidic environment. The pH sensitive material 610 a isopaque so that when present, as depicted in FIG. 6A, infrared lighttransmitted from the IR transmitter 608 a is not received at thereceiver 608 b. When the pH sensitive material 608 b is degraded asdepicted in FIG. 6B, infrared light transmitted from the IR transmitter608 a is received at the IR receiver 608 b as depicted by arrows 612.The detection of the infrared light at the receiver 608 b may bedetected by the electronics of the tag and used as an indication that apredation event has occurred.

FIG. 7 depicts a further predation tracking tag. The tag 700 may besimilar to the tags described above, however the body of the tag isextended to provide physical protection to the predation sensors. Thepredation tags described above may include a predation sensor thatextends past the body of the tag. Due to the small size of the tags, theextending predation sensors may be susceptible to breaking. Thepredation tag 700 comprises a body that provides protection to thepredation sensor 704. The body comprises a main body portion 702 a thatencases the electronics 712 of the tag. The main body portion 702 a maybe provided by a tube or other similar structure. The body furthercomprises a hollow extending portion 702 b that extends past thepredation sensor 704 and provides physical protection to the predationsensor against damage. The hollow cavity of the extending body portion702 b may be filled or partially filled with pH sensitive material 710a. The extending portion 702 b of the tag body is open at one end inorder to expose the pH sensitive material to the environment of the tagso that the pH sensitive material will degrade when a predation eventoccurs.

FIGS. 8A, 8B, 8C, 8D, 8E, 8F depict further predation tracking tags. Thetag 800 is similar to the tag 200; however, rather than using electrodesto detect a change in an electrical characteristic, the tag 800 uses amagnetic sensor 810 to detect the presence or absence of a magnet 806.As depicted in FIG. 8A, the tag 800 includes a body 802 that has arecessed portion 804 for receiving a magnet 806. Although depicted asbeing situated within a recess 804, it is contemplated that the magnetmay be affixed to the tag without being situated within a recess. Themagnet 806 is affixed to the tag 800 by a pH sensitive material 808 a asdepicted in FIG. 8B. The magnet 806 may be affixed by a layer of pHsensitive material that acts as an adhesive to hold the magnet 806 inplace while a further coating of pH sensitive material is used tofurther adhere the magnet 806 to the tag 802. As previously described,the pH sensitive material degrades in the acidic environment of apredator's gut. Once the acidic environment degrades the pH sensitivematerial 808 b that affixes the magnet 806, the magnet 806 may separatefrom the tag 800 and the absence of the magnet may be detected by thetag 800. Once the absence of the magnet 806 is detected by the tag 800,the tag may switch modes of operation to indicate that a predation eventhas been detected.

The magnet and pH sensitive material may be attached to the tag body inother manners than described above. For example, a further tag 800 d isdepicted in FIG. 8D. As depicted, the magnet 806 may be encased in aplug 808 d made of the pH sensitive material, and the plug 808 d may beattached to a body 802 d in various ways, including for example by wayof an adhesive material 812.

A further example of a tag 800 e is depicted in FIG. 8E. As depicted,the magnet 806 may be encased in plug 808 e of the pH sensitivematerial. The plug 808 e is shaped to allow the plug to be mechanicallyretained within a correspondingly shaped cavity portion 814 e of the tagbody 802 e, until at least a portion of the pH sensitive material isdissolved. A still further example of a tag 800 f is depicted in FIG.8F. As depicted, the magnet 806 may be encased in plug 808 f of the pHsensitive material; however, rather than being retained in acorresponding cavity of the tag as described above, the plug 808 fcomprises a corresponding cavity that mechanically retains acorrespondingly shaped protruding portion 814 f of the tag body 802 f.

The tags 800, 800 d, 800 e, 800 f described above rely on the magnetseparating from the tag. While such separation may occur, it may bedesirable to provide an additional force to aid in the separation, or atleast movement of the magnet, to change the magnetic field detected bythe sensor. Such a force may be provided by including a magneticattractor in the tag. The magnetic attractor provides a force to themagnet which will tend to result in movement of the magnet upon partialor full degradation of the pH sensitive material. Although the magneticattractor is described as providing an attractive force, it will beappreciated that a repulsive force may be provided and may have the sameeffect. That is, both attraction and repulsion of the magnet may beprovided by the magnetic attractor. Implementation of a tag comprising amagnetic attractor is described in more detail with respect to FIGS.9-11.

FIGS. 9A through 9D depict a further predation tag 900. FIGS. 9A through9D may be referred to collectively as FIG. 9 where such reference doesnot cause any ambiguity or lack of clarity. As depicted, a first magnet906, which may be referred to as a sensing magnet, is encased, or atleast partially encased, in the pH sensitive material 908 a. The sensingmagnet 906 and the pH sensitive material 908 a may form a plug that isshaped such that it is mechanically retained within a correspondinglyshaped cavity portion 904 of the tag body 902. A magnetic field sensor910 is situated beneath cavity portion 902, and is oriented such that anaxis of sensitivity of the sensor 910 is parallel, or at least generallyparallel, to the magnetic field of sensing magnet 906. As such, when thesensing magnet 906 is arranged with its magnetic field aligned with thesensor's axis of sensitivity as depicted in FIGS. 9A and 9B, the sensordetects the magnetic field. The magnetic attractor, which provides aforce tending to move the sensing magnet when it is not retained by thepH sensitive material, may be provided by a second magnet 912. Thesecond magnet 912, which may be referred to as an attraction magnet, maybe embedded within the tag body. The second magnet 912 may bepermanently secured to the tag body 902 in close proximity to cavityportion 904 so that the magnetic fields of the second magnet and thesensing magnet interact with each other and exert forces on the magnets.As depicted in FIG. 9 the second magnet 912 may be positioned such thatits magnetic field is perpendicular, or at least generallyperpendicular, to the magnetic field of the sensing magnet. With themagnetic field of the sensing magnet 906 generally aligned with the axisof sensitivity of magnetic field sensor 910, the magnetic field of thesecond magnet 912 does not substantially affect the sensor. Thisalignment of the sensing magnet 906 with the axis of sensitivity ofmagnetic field sensor 910, plus the perpendicular positioning of themagnetic field of the second magnet 912 relative to the magnetic fieldof the sensing magnet, works to prevent detection of the magnetic fieldof the second magnet 912 by the sensor 910. Accordingly, prior to apredation event, with the sensing magnet 906 encased in the pH sensitivematerial as depicted in FIGS. 9A and 9B, the sensor 910 detects themagnetic field of the sensing magnet 906 that is at least generallyparallel to the sensor's axis of sensitivity.

During a predation event, the pH sensitive material will degrade. Forexample, as depicted in FIGS. 9C and 9D, the pH sensitive material 908 bis degraded as a result of a predation event. Once the pH sensitivematerial has sufficiently degraded, the sensing magnet 906 may bereleased from the pH sensitive material of the tag and may become loosewithin cavity 904. With the sensing magnet released, magneticinteraction between the sensing magnet 906 and second magnet 912 willcause the sensing magnet 906 to move or rotate such that it becomessubstantially parallel to second magnet 912 as illustrated in FIGS. 9Cand 9D. This results in the magnetic field of sensing magnet 906becoming perpendicular to the axis of sensitivity of magnetic fieldsensor 910, thereby causing the sensor to no longer detect the presenceof a magnetic field. The stationary second magnet 912 provides amagnetic attractor that tends to cause the sensing magnet 906 to rotatewhen released from the pH sensitive material 908 b and align itsmagnetic field with that of the second magnet 912, thereby causing adetectable change of the magnetic field detected by the sensor 910.

It will be appreciated that the orientation of one or more of thesensor, sensing magnet and magnetic attractor may be varied. Forexample, one or more of the sensor, sensing magnet and magneticattractor may be arranged so that the magnetic field of the sensingmagnet is initially perpendicular to the axis of sensitivity of thesensor, resulting in the sensor not sensing the presence of the sensingmagnet, and subsequently upon degradation of the pH sensitive materialparallel to the axis of sensitivity of the sensor, resulting in thesensor sensing the presence of the sensing magnet and thereby sensing apredation event. However, in such an embodiment, the second magnet mayneed to be located sufficiently far from the sensor so that the strengthof its magnetic field is below a detection threshold at the sensor,while still providing a force on the sensing magnet. Additionally oralternatively, the sensor may detect a strength of the magnetic field,or changes in the strength of the magnetic field.

FIGS. 10A through 10D depict a further predation tag 1000. FIGS. 10Athrough 10D may be referred to collectively as FIG. 10 where suchreference does not cause any ambiguity or lack of clarity. The tag 1000is similar to the tag 900 described above with reference to FIG. 9 inthat the tag 1000 includes a magnetic attractor for causing, or at leasttending to cause, movement of the sensing magnet 1006 when released fromthe pH sensitive material 1008 a. However, in contrast to the tag 900which includes a second magnet 912 that tends to cause the sensingmagnet 906 to rotate to align its magnetic field axis with that of thesecond magnet 912 upon degradation of pH sensitive material, themagnetic attractor of the tag 1000 comprises a ferromagnetic material1012 that the sensing magnet is attracted to. Although depicted as aferromagnetic piece of material, the magnetic attractor 1012 maycomprise a second magnet that the first magnet would be attracted to.Further, rather than detecting an alignment, or mis-alignment, of themagnetic field, the sensor 1010 may detect the presence or absence of amagnetic field, or a magnetic field above a threshold. As depicted, themagnetic sensor 1010 is aligned under a cavity 1004 within the body 1002of the tag 1000. The sensor 1010 is positioned to detect the presence ofthe sensing magnet 1006 when the sensing magnet 1006 is located in afirst location depicted in FIGS. 10A and 10B. The sensing magnet 1006may be secured within the cavity 1004 in the first location by the pHsensitive material 1008 a.

During a predation event, the pH sensitive material 1008 b is degradedsufficiently to release the sensing magnet 1006. When released, thesensing magnet 1006 may be loose within cavity 1004. The sensing magnet1006 is attracted to the ferromagnetic material 1012 of the magneticattractor and as such moves towards the ferromagnetic material 1012. Themovement of the sensing magnet 1006 causes it to move out of the firstlocation where the magnetic sensor 1010 detects its presence and into asecond location where the sensing magnet 1006 is not detected by themagnetic sensor 1010. The ferromagnetic material 1012 provides amagnetic attractor that tends to cause the sensing magnet 1006 to movewhen released from the degraded pH sensitive material 1008 b, therebyresulting in a detectable change of the magnetic field detected by thesensor 1010.

FIGS. 11A and 11B depict a further predation tag 1100. FIGS. 11A and 11Bmay be referred to collectively as FIG. 11 where such reference does notcause any ambiguity or lack of clarity. The tag 1100 includes a firstmagnet 1106, which may be considered at least conceptually as a sensingmagnet. The tag 1100 further includes a second magnet 1112, which may beconsidered at least conceptually as an attraction magnet.

However, in contrast to the tag 900 in which the attraction magnet 912is permanently fixed to the tag body, the second magnet 1112 is notpermanently fixed to tag body 1102. Rather, the second magnet 1112 issecured or located against or adjacent to first magnet 1106 by the pHsensitive material 1108 a. The first magnet 1106 and the second magnet1112 are secured to each other with their respective magnetic fieldsaligned, that is like poles are adjacent each other, so that themagnetic fields add constructively, thereby resulting in a relativelylarge magnetic field detected by sensor 1110. As depicted in FIG. 11A,when secured by the pH sensitive material 1108 a, the first magnet 1106and second magnet 1112 are aligned with their north poles adjacent andsouth poles adjacent.

During a predation event, the pH sensitive material degradessufficiently to allow the second magnet 1112 to rotate relative to thefirst magnet 1106, or vice versa. The initial alignment of the magnetsand their corresponding magnetic fields will tend to cause the magnetsto ‘flip’ relative to each other so that the first magnet's south poleis adjacent the second magnet's north pole, and the first magnet's northpole is adjacent the second magnet's south pole. With the opposite poleslocated adjacent each other, the respective magnetic fields of the firstmagnet 1106 and the second magnet 1112 will add destructively resultingin a smaller magnetic field being detected by the sensor 1110.Accordingly, the second magnet 1112 causes, or at least tends to cause,movement of the first magnet 1106, and vice versa, thereby causing adetectable change in the magnetic field. It will be appreciated thatmore than two magnets may be employed in a variation of theabove-described embodiment.

The first and second magnets may be indistinguishable from each other.However, in combination, each acts as a sensing magnet and as anattraction magnet to the other magnet. The use of a plurality ofindividual magnets secured within a pH sensitive material with theirmagnetic fields aligned such that their magnetic fields addconstructively may be applied to magnetic particles embedded within thepH sensitive material in order to provide a degradable magnet. Themagnetic particles may be embedded within the pH sensitive material andan external magnetic field used to magnetize the embedded particles sothat their magnetic fields are aligned and add constructively to providea magnetic field that is detectable by the sensor. When the pH sensitivematerial is degraded by the acidic environment, the magnetic particlesare released and free to rotate or otherwise move relative to each otherand as such, will exert forces on the other magnetic particles such thatthe magnetic fields will align to cancel out and result in no netmagnetic field.

Various tags have been described above that can be used to track amarine animal and also provide an indication that the animal has beeneaten by a predator. Although particular embodiments have been describedwith reference to tags for tracking marine animals, it is possible for asimilar tag to be used to track other animals. In the case of trackingnon-aquatic animals, the ultrasonic signals may not be useful and assuch the transducer of the tag may be replaced with an appropriatetransducer of signals that may be detected and used for tracking thenon-aquatic animal. For example, the transducers may use radio frequency(RF) signals rather than the ultrasonic signals used for marineenvironments.

The tags described above with reference to FIGS. 9, 10, 11 each comprisea predation detector that comprises a magnetic sensor, a first magnet atleast partially encased in pH sensitive material and a magneticattractor arranged such that upon degradation of the pH sensitivematerial the first magnet is free to move and the magnetic attractorprovides a force that tends to cause the magnet to move. The movement ofthe first magnet results in a change in the magnetic field sensed by thesensor. When the electronics of the tag, or other circuitry, detect achange in the magnetic field, or a magnetic field that meets othercharacteristics, such as the presence or absence of the magnetic field,a predation event may be triggered. The predation detection describedabove may be suited for incorporation into a wide range of animaltracking tags, whether for tracking aquatic, land or airborne animals.The tags may telemeter data from various other sensors and may transmit,store and/or process the information in various ways, which may beaffected by detection of a predation event. The transmission ofinformation from the tag may use different transducers such as acoustictransmitters, active radio frequency transmitters and/or passive radiofrequency transmitters such as RFID tags. The logging or storing of theinformation may be to volatile or non-volatile memory of the tag forsubsequent analysis if the tag is retrieved.

With non-aquatic animals, a tag is often attached externally rather thanimplanted internally into the coelomic cavity. In such embodiments thetag with the predation detector may benefit from protection againstimpact, abrasion and UV light, since these may degrade the pH sensitivematerial potentially causing the tag to falsely detect a predationevent. This protection could be accomplished by mechanical shielding.The effects of external liquids (e.g. rain, ground water, etc) on thepredation detector will depend on their specific chemistry at the site.Such characteristic may need to be considered to determine if thepotential study conditions are suitable to avoid false detection ofpredation events.

Applications to non-aquatic animals may be practically limited to smallanimals that are more-or-less fully consumed by a large predator suchthat the tag is actually swallowed intact. This would be the case, forexample, when a snake consumes its prey whole. In other cases where thepredator only partially consumes the prey or eats it a bit at a time, itmay not swallow the tag at all or may damage it. For medium to largeanimals, tags are typically attached by means of a collar made of somecombination of metal, plastic and leather, and a predator is not likelyto eat such a collar.

The tags described herein are battery operated and are generallyactivated prior to being tagged to an animal. Tags may include amagnetic sensor in order to provide a means of activating the tags. Thetags may be inserted into a magnetic field produced by an activationdevice. The magnetic sensor of the tag detects the magnetic field, whichmay provide instructions for activating the tag.

FIG. 12 depicts an activator for activating a tracking tag. Theactivator 1200 may be handheld device into which a tag 1202 to beactivated is inserted. The activator 1200 has a chamber 1204 into whichthe tag can be placed. An electromagnetic coil may be wound around thechamber 1204 so that when the tag is placed within the chamber 1204, itis within a magnetic field of the coil. The magnetic field produced bythe coil can be controlled by the activator 1200 and may communicateactivation commands to tags.

It may be possible to communicate other information from the activatorto the tag by controlling the magnetic field produced by the coil. Forexample, an operation mode of the tag may be set, or transmissionfrequency or other options may be controlled. The activator may alsode-activate tags in a similar manner. The activator 1200 includes inputand output components for controlling the operation of the activator.For example, the activator may have an ‘On’ button 1206 that is used foractivating the tag. An ‘Off’ button 1208 may be used to deactivate anactivated tag. The activator 1200 may include outputs or display meansfor providing information about the tag. For example, the activator mayinclude an ‘Off’ LED 1210 that may be used to indicate that the tag isdeactivated. A ‘Busy’ LED 1212 may indicate that the activator iscurrently busy, for example it may be attempting to activate ordeactivate a tag. An ‘On’ LED 1214 may indicate that the tag has beensuccessfully activated. In order to determine if the tag has beensuccessfully activated or deactivated, the activator 1200 may listen forthe ultrasonic transmission of the tag.

FIG. 13 depicts components of the tracking tag of FIGS. 2A-C and anactivator. As described above, an activator may be used to activate atag. A tag 1302 may comprise a power supply 1304, such as a battery, amicrocontroller 1306, or other type of circuit for controlling theoperation of the tag 1302. The tag 1302 may also include memory 1308 forstoring instructions and/or data. Although depicted as being a separatecomponent from the microprocessor 1306, the memory 1308 or at least aportion of the memory 1308 may be part of the microcontroller 1306. Thetag may further comprise an ultrasonic transducer 1310 for transmittinginformation, such as the unique ID, when the tag is activated. The tag1300 may also include a magnetic sensor 1312 that is used to receiveinstructions from the activator. Further, as described above, the tag1302 may include a predation sensor 1314 for measuring a characteristicof the tag that will change when in an acidic environment.

The activator 1316 includes a power supply 1318, a microcontroller 1320for controlling operation of the activator as well as memory 1322 forstoring instructions and/or data used by the microcontroller 1320. Themicrocontroller 1320 may control an electromagnetic coil 1324 in orderto generate a magnetic field used to communicate commands to the tagwhen the tag is located within the generated magnetic field. Thecommands may include commands for activating and/or deactivating the tagas well as for setting operational parameters of the tag. The activator1316 may include a microphone 1326 for detecting the ultrasonic pulsestransmitted by the tag once activated. Although not depicted, theactivator may include a display for displaying the unique ID of the tag.The activator 1316 may include input/output components 1328 in order toallow a user to interact with the activator 1316 to activate andde-activate, as well as possibly configure tags.

FIG. 14 depicts components of a predation tracking tag and an activator.The tag 1402 is similar to the tag 1302 described above with referenceto FIG. 13 and as such only the differences are depicted. Also, theactivator 1408 is similar to the activator 1316 and as such only thedifferences are depicted. The tag 1402 uses the magnetic sensor 1404 todetect the presence or absence of a magnet 1406. As described above, themagnet 1406 can be affixed to the tag using the pH sensitive material.While the magnet 1406 provides a convenient means for detecting apredation event, its presence near the magnetic sensor 1404 mayoverwhelm the magnetic field generated by the activator and make theactivation process difficult. The activator 1408 may include magneticfield compensation functionality 1410 for overcoming the magnetic fieldof the magnet 1406 in order to communicate with the tag. The magneticfield compensation functionality 1410 may generate a varying magneticfield in order to determine the magnetic field required to cancel, orcounteract, the magnetic field of the magnet 1406. Once the activatordetermines the magnetic field required to compensate for the presence ofthe magnet 1406, it can be used as a base line in varying the magneticfield to allow communication between the activator 1408 and tag 1402.With the magnetic field of the magnet 1406 cancelled at the sensor 1404by the base line magnetic field compensation, the magnetic sensor 1404can detect changes in the magnetic field caused by the activator, and assuch the activator may communicate with the tag 1402.

Once a tag is activated it may be implanted into a coelomic cavity of amarine animal, or otherwise attached to the animal, and the animal maybe released and tracked using the tag.

FIG. 15 depicts components of a magnetic predation tracking tag and anactivator. The magnetic predation tracking tag may be, for example, atag such as the tags of FIGS. 8A-F, FIGS. 9A-D, FIGS. 10A-D and FIGS.11A-B. The activator 1500 comprises a microcontroller 1502 that controlsa number of digital to analog converters (DACs) 1504 a, 1504 b (referredto collectively as DACs 1504) and a Universal AsynchronousReceiver/Transmitter (UART) 1510. A reference voltage 1506 is suppliedto the DACs 1504, whose outputs are provided to an analog multiplexer1508. The output of the analog multiplexer 1508 is controlled by theUART 1510 and supplied to a coil driver 1512 to produce a magnetic field1514. In order to receive information from the tag 1530, the activatorincludes an acoustic transducer 1518 that detects acoustic signals 1516from the tag. The output of the acoustic transducer 1518 is provided toan amplifier 1520 to amplify the signal which is provided to a signaldetector 1522 and the results supplied to the microcontroller 1502.

The tag 1530 includes a magnetic field sensor 1534. The magnetic fieldsensor 1534 may use a GMR (giant magneto resistive) sensor, butalternative designs could use a mechanical reed switch, Hall effectsensor or other type of sensor. Typically, these sensors have a digitaloutput that will output a “1” if an external magnetic field is present,and a “0” if no magnetic field is present, or vice versa. Typically aminimum magnetic field level is required in order for these sensors toindicate that a field is present. The magnetic field sensor provides itsoutput to a UART 1536, which provides it to a microcontroller 1538.Alternatively, the magnetic field sensor output may be provided to themicrocontroller 1538. The microcontroller processes the magnetic fieldinformation in order to control operation of the tag. Themicrocontroller may output information to a signal generator 1540 whichgenerates a signal that is amplified by an amplifier 1542 and providedto an acoustic transducer 1544 that produces an acoustic signal 1516. Asdescribed above, the acoustic signal 1516 may be detected by theactivator 1500.

When the predation detection magnet 1532 is attached to the tag 1530,the magnetic sensor 1534 will detect the field of the magnet 1532 andwill indicate the presence of the magnetic field in its output. Withoutadditional magnetic fields applied, the output will remain the sameuntil the magnet is removed, either manually or by a predation event.Unfortunately this means that sending data into the tag becomes moredifficult since simply varying a magnetic field will not be detected. Inorder to send data into the tag 1530, the activator 1500 has to apply amagnetic field with equivalent strength and opposite polarity, so thatthe magnetic sensor 1534 detects that no field is present. That is, theactivator must apply an opposite magnetic field in order to counter actthe magnetic field of the magnet.

The activator may generate a fixed magnetic field to counteract a knownmagnet strength. However, generating a fixed magnetic field may not bereliable since the magnetic field to be countered may vary from tag totag. The magnet attached to the tag could be affixed in two differentpolarities as the magnets are relatively small at approximately 1×0.5 mmand don't have the poles labeled. Determining the orientation of thepoles during assembly would be difficult. Further, the magnetic field ofthe magnet may have some variation on its field strength due tomanufacturing/material tolerances. Additionally, the distance betweenthe magnet and the magnetic field sensor may vary, due to mechanicalvariation in the construction of the case, thickness of base pHsensitive film, and positioning variation when the product assemblerattaches the magnet to the tag. Finally, the customer using the tagactivator could insert the tag into the activator in two possibleorientations. The overall effect is that it is impractical to apply afixed magnetic field level to the tag which would reliably cancel thepredation magnet field and permit reliable data communication.

As described further below, it is possible to calibrate the activator inorder to counter act the magnet's field and so allow communicationbetween the activator and the tag. The activator may configure one ofthe DACs 1504 a to provide an output that will generate the counteracting magnetic field and so cause the magnetic field sensor 1534 of thetag to indicate that no field is detected. The second DAC 1504 b may beconfigured to provide an output that will generate a complementary fieldto cause the magnetic field sensor 1534 of the tag to indicate that afield is detected. The UART may then control the analog multiplexer inorder to select which of the signals is used to drive the coil 1512 andso generate the magnetic field.

FIG. 16 depicts a method of activating a tag. When a tag is insertedinto the activator, the tag is placed in a calibration mode in order todetermine the counter acting magnetic field. Once the calibration isperformed, data can be transferred from the activator to the tag. Themethod begins with generating a wakeup signal (1602). The wakeup signalmay be a rapidly varying magnetic field. For example a triangular 100 Hzwaveform may be generated. The varying magnetic field is detected at thetag (1604). The varying magnetic field will at certain points counteractthe magnet's field and as such cause a change in the detected magneticfield. In normal operation, the magnetic field will not vary quicklyover time and as such, when the tag detects the varying field, the tagcan enter a calibration mode (1606). The tag may indicate that it hasentered into the calibration mode (1608) to the activator by sending anappropriate pulse train from the ultrasonic transducer. The activatorreceives the indication that the tag has entered the calibration mode(1610) and begins varying the magnetic field (1612) which is detected atthe tag (1614). The tag indicates the presence or absence of a detectedmagnetic field (1616) to the activator. The activator determines thecounter-acting magnetic field (1618) that caused the tag to detect nomagnetic field and sets the DACs (1620) used to generate the magneticfields representative of ‘0’ and ‘1’. The tag enters the data transfermode (1622) and the activator begins to transfer data (1624) which isreceived at the tag (1626).

FIG. 17 depicts signals associated with activating a tag. Initially, theactivator first drives the coil with a full-scale triangle wave at afrequency of 100 Hz as depicted FIG. 17. This results in a fairlydistinctive “double pulse” signal from the magnetic field sensordepicted in FIG. 17. The tag detects the double pulse signal and entersthe calibration mode. Detection of the waveform in the tag can beaccomplished by determining if the magnetic field sensor value isdifferent from a previous sensed value, and if it is different keep themagnetic field sensor on and observe the sensed value for the next 20 msor more. If the magnetic field sensor values appear to be rapidlychanging, the magnetic field sensor output is observed for a fixedperiod of time and the number of transitions in the value is counted. Ifthe frequency is within the expected 200 Hz range (two transitions foreach triangular pulse at 100 Hz) the tag enters the calibration mode.Upon detecting the varying signal, the tag pings out a distinctivesequence of pings, which are different from its usual operatingtransmissions, to tell the activator that it's entering calibrationmode. Although depicted as a triangular waveform having a frequency of100 Hz, it is possible for the wave form to have different frequenciesor shapes. For example, the signal may have frequencies ranging fromapproximately 10 Hz, or less, to approximately 10 kHz or more. Thefrequency range of between 10 Hz and 10 kHz is only illustrative, andthe frequency may be greater than 10 kHz or less than 10 Hz.

Once the tag is in the calibration mode, the activator attempts todetermine the DAC setting required for counteracting the magnet'smagnetic field. In the calibration mode, the tag keeps its magneticfield sensor powered on. If the magnetic field sensor detects theabsence of a magnetic field, the tag will continuously ping, otherwisethe tag will remain silent. For accuracy, the tag may ping immediatelyafter the magnetic field sensor detects the absence of a magnetic field.

If the magnetic field sensor is held low for a sufficiently long anduninterrupted time, then the tag enters data transfer mode. If themagnetic field sensor does not go low for a long period of time, it isassumed that a fault occurred in the activation process and the tag maygo back to regular operation.

In calibration mode, the activator may search for the “zero point”, thatis the DAC setting that counteracts the magnet's magnetic field at thesensor by the following method of the following pseudo code. In thefollowing, it is assumed that ‘DAC0’ is the first DAC used forgenerating the counteracting magnetic field and that the DAC values canvary between 0 and 4095. A value of 0 corresponds to full scale negativecurrent, 2048 to zero current, and 2095 to full scale positive current.

-   -   Start with the magnetic field at zero (DAC0=2048)        -   If the tag pings, there's no magnet present—set DAC0 to 2048            and return;    -   Increase the DAC0 setting until:        -   DAC0=4095 is reached;        -   Or the tag pings;    -   If the tag pinged:        -   Record the DAC0 setting;        -   Set the DAC0 value to 4095;        -   Slowly decrease the magnet field (decrement DAC0 setting)            until the tag pings;        -   Record the second DAC0 setting;    -   If DAC0 reached 4095:        -   Set the magnetic field back to zero (DAC0=2048);        -   Decrease the DAC0 setting until the tag pings; Record the            DAC0 value;        -   Set the DAC0 value to 0;        -   Increase the DAC0 value until the tag pings. Record the DAC0            value;    -   Set the DAC0 value to the average of the two recorded values.

The activator then holds the ‘DAC0’ value, until the tag stops pinging.Once the counteracting magnetic field is calibrated, the tag may beplaced in a data transfer mode in which the activator sets the first DACto the ‘DAC0’ value calculated and sets the second DAC to a suitablevalue ‘DAC1’ that will ensure that the magnetic sensor detects amagnetic field. The value may be determined according to:

-   -   If (DAC0<2048) then DAC1=4095; (full scale positive)    -   Else DAC1=0 (full scale negative)

Once the DACs are set, the data can be transmitted to the tag byswitching the multiplexer output to select which DAC drives the coil.

FIG. 18 depicts further signals associated with activating a tag. Asdescribed further with reference to FIG. 18, it is possible to determinethe DAC setting required for counteracting the magnetic field of themagnet at the tag. It is possible to combine the activation andcalibration waveforms into a single step—the tag observes the waveformcoming out of the magnetic field sensor, calculates a calibration valueand reports the calculated value to the activator.

This method requires the magnetic field sensor to be detected with ahardware timer in the tag's microcontroller, or with fast running codeon the tag's microcontroller, in order to measure time delays withreasonable accuracy. To wake up the tag, the activator applies anasymmetrical, saw-tooth waveform to the coil as depicted in FIG. 18. Thecoil driving signal has a rise time of approximately 3.33 ms, a falltime of approximately 6.67 ms, and a period of 10 ms (100 Hz). Althoughdepicted as a triangular waveform having a frequency of 100 Hz, it ispossible for the wave form to have different frequencies or shapes. Forexample, the signal may have frequencies ranging from approximately 10Hz, or less, to approximately 10 kHz or more. The frequency range ofbetween 10 Hz and 10 kHz is only illustrative, and the frequency may begreater than 10 kHz or less than 10 Hz. Further, the rise and fall timesmay vary based upon the asymmetry of the waveform.

The resulting signal coming out of the magnetic field sensor, depictedin FIG. 18 is similar to the previous “double pulse” waveform, exceptthat the two pulses have different lengths. Whether the narrow or widepulse comes first depends on the magnetic field polarity required tocancel the predation magnet. Once the tag determines the presence of thewakeup signal, the tag measures the following delays:

-   -   T1 Width of first pulse    -   T2 Width of second pulse    -   T12 Time delay from end of the first pulse to start of the        second pulse

Given these values, the tag can calculate the required calibration valuewith the method of the following pseudo code:

If (T2 > T1) { DACH = 4095 − (T12 * k1); DACL = DACH − (T2 * k2); DAC0 =(DACH + DACL) >> 1; } else { DACL = (T12 * k1); DACH = DACH + (T1 * k2);DAC0 = (DACH + DACL) >> 1; }k1 and k2 are constants derived from the waveform frequency, waveformduty cycle and the units of measure of T1, T2 and T12.

The resulting ‘DAC0’ value, which may vary between 0 and 4095 forexample, is pinged back to the activator using a distinct PPM sequence,different from a usual PPM ping. The tag may then enter data transfermode.

The activator then sets its ‘DAC0’ value to the provided value, sets‘DAC1’ to a suitable complementary value, and begins data transfer.

FIG. 19 depicts a method of operating a tracking tag. The method 1900begins when a tag is activated (1902) and implanted into a coelomiccavity of an animal. The animal may then be released back into the bodyof water, with the tag operating to periodically transmit its unique ID.The tag transmits its ID and any other additional data (1904), such assensor information including temperature sensors, acceleration sensorsor any other sensors. Once the ID has been transmitted the tag waits(1906) for a period of time. The period of time that the tag waits formay vary. However, once the wait period is over, the tag determines if adetected characteristic crossed a predation threshold (1908). Thepredation threshold provides an indication of detected characteristicvalues that are associated with the tag being in an acidic environment.If the detected value has not crossed the predation threshold (No at1908) the tag transmits its ID and any additional data again (1904). Ifthe detected value has crossed a predation threshold (Yes at 1908), apredation event has occurred, and as such the tag may change itsoperating mode. The tag may begin to transmit a predation ID (1910),which differs from the regular tag ID, in order to indicate that apredation event has been detected. The tag may also transmit additionaldata such as temperature readings or values from other sensors. Theadditional data may also include an indication of the amount of timethat has elapsed since the detection of a change in the characteristicof the tag, which change is associated with the predation event. After apredation event has been detected, it is no longer necessary to check ifthe predation threshold has been crossed, and as such the tag may wait(1912) for a period of time before transmitting the predation ID again(1910).

The predation tags described above utilize a pH sensitive material thatdegrades preferably quickly in an acidic environment, but does notdegrade in a neutral or basic environment. As described further, variouspH sensitive materials were developed and tested for theirappropriateness in use with predation tags. The pH sensitive materialshould be robust enough to survive months inside the body cavity of afish where the tag is implanted, yet break down relatively rapidly oncepredation occurs.

Chitosan was used in creating pH sensitive material. The chitosan pHsensitive material may be formed from casting a slurry and evaporating asolvent of the slurry. Swelling and degradation properties of thechitosan can be affected by different additives such as plasticizers andcross-linkers. Results of tests done on various chitosan slurries aswell as the use of the slurry on tags are set out further below.

Medium molecular weight (approximately 750,000 Daltons) chitosan wereused in the preparation and testing of the pH sensitive material. Thechitosan was purchased from Sigma Aldrich™. It is low cost with 88%deacetylation, as determined via NMR analysis. The chitosan can bedissolved in solution and then cast as a film or used as an adhesive.The properties of chitosan and its film can be tailored based on thedegree of deacetylation, crystallinity, purity, molecular weight of thesample, pH of the environment, presence or absence of plasticizingagents, crosslinking, drying and isolation conditions, and the acid usedin the solvent. Various slurries of chitosan were tested to determinethe rate of degradation in simulated coelomic fluid, the degree ofswelling in simulated coelomic fluid, and the time to break down in anacidic solution. It is desirable that the material be stable in theventral body cavity of a fish yet quickly dissolve in the primitivestomach of predators.

Chitosan can be dissolved in a protic solvent to obtain a uniform slurrythat can be cast as a film or utilized as an adhesive. In the case offilm formation, the solvent is allowed to evaporate over a period oftime. The composition of the solution, presence of additives, andevaporation conditions have all been shown to effect the properties ofthe resultant product.

The most influential factor on the properties of the resultant productis the choice of solvent mixture. Four acidic aqueous solutions wereconsidered for solvation of the chitosan. The solutes includedL-ascorbic acid, citric acid, acetic acid, and hydrochloric acid. Theresults of the solute testing are presented in Table 1.

TABLE 1 Effect of various solutes on chitosan film formation. SoluteTest Results L-ascorbic Films showed evidence of extreme decompositionover acid time in air. Citric acid Films showed good adhesion andtoughness when wet. Films showed excess swelling when placed in waterand contracted upon drying. Films were not flexible when dry. Aceticacid Films did not adhere well. Less noticeable swelling when wet.Slight wrinkling of edges while drying. Films maintained a good degreeof flexibility when dry. Hydrochloric Films were brittle. Higherconcentration of HCl caused acid hydrolysis.

As can be seen from table 1, acetic acid and citric acid showed the mostfavorable results and were used for further testing. The use of aplasticizing agent can improve the characteristics of the materials castwith acetic acid as described further below. Citric acid materials didnot show improved characteristics with plasticizer and as such slurrieswith acetic acid were focused upon.

Adhesion of films and slurry to substrate materials such as epoxy andparylene may be important to ensure that the film remains adhered to thetag. Studies of chitosan slurries with acetic acid focusing on adhesionof the film on a substrate were performed. While drying, often the filmwould show poor adhesion and peel up. It was found that the adhesion ofthe film to the substrate could be improved by sanding the surface ofthe substrate with 600 grit sandpaper. The sanding increased the surfacearea and allowed for strong adhesion, with no resulting peeling orcurling at the edge of the films. All films passed a scratch and tapetest once the surfaces had been sanded. A scratch test is simplyscratching the surface of the film with a fingernail, while the tapetest was done by adhering a one inch piece of STAPLES™ brand scotch tapeand removing it quickly. A test was considered positive if the filmremained intact after the test. Chitosan films were tested for theiradhesion to a parylene coated surface and epoxy pucks. The chitosanshowed good adhesion to the parylene. After sanding, the chitosan filmshowed good adhesion to the epoxy pucks. Generally, good adhesion wasobserved after the surface area was increased by sanding and all passedthe scratch and tape tests.

The films of chitosan cast in acetic acid were flexible and strong. Inorder to test the swelling and degradation characteristics of the filmsas well as the effect of additives on the films, the films were testedin simulated coelomic fluid (SCF). The SCF was made to mimic theconditions inside the body cavity of a fish that are typical of marinefish. The SCF was composed of: 0.02 mM HEPES buffer solution (83264-500ML, Sigma Aldrich), 124.1 mM NaCl (Sigma Aldrich), 5.1 mM KCl (SigmaAldrich), 1.6 mM CaCl2.H₂O (Sigma Aldrich), and 1.0 mM MgSO₄.H₂O (JTBaker). The pH of the solution was adjusted to 8.20 using 1 M NaOH.Solutions used in biological studies also often include dextrose andpenicillin. These were omitted here as they are considered to havelittle influence on the ionic conductivity and degradation of the films.Also, dextrose could allow bacteria to grow on the samples duringlong-term studies.

The films were allowed to sit in 5 mL of the SCF for different periodsof time. During these periods the solutions were agitated daily and thefluid was replaced every three days to imitate the natural replenishmentof the coelomic fluid in a living fish.

Before the films were placed in the SCF they were weighed to get theirinitial dry mass. After allowing them to sit in the SCF for a specifiedamount of time they were removed from the fluid. They were patted dryand weighed immediately to get the wet mass. The films were then allowedto dry for the period of a week and then weighed again to get the drymass after the trial. Using these three different masses (dry mass, wetmass, and dry mass after trial), the swelling and degradation of thefilms were quantified, as follows:

$\begin{matrix}{{\% \mspace{14mu} {swelling}} = {\frac{{{wet}\mspace{14mu} {mass}} - {{dry}\mspace{14mu} {mass}}}{{dry}\mspace{14mu} {mass}} \times 100}} & (1) \\{{\% \mspace{14mu} {degration}} = {\frac{{{dry}\mspace{14mu} {mass}\mspace{14mu} {after}\mspace{14mu} {trial}} - {{dry}\mspace{14mu} {mass}}}{{dry}\mspace{14mu} {mass}} \times 100}} & (2)\end{matrix}$

Equation (1) shows the swelling experienced by a film in the SCF whileequation (2) shows the mass loss of the film, which is referred to asdegradation of the same film.

A value of 100% swelling would represent a film that has doubled itsmass and therefore experienced considerable swelling. It is not unusualfor these thin films to take up a lot of water while in the SCF. A valueof 0% swelling means that that change in mass was undetectable, but 0%swelling is not characteristic of these chitosan films. The degradation% is usually expressed as a negative, meaning that mass was lost duringstudies. A value of −50% degradation would signify that the film haslost half of its original mass. Typical values are around −10%degradation.

It was observed that the thickness of the films may drastically affectthe swelling and degradation of the films, as seen in the graph of FIG.20. Films of differing thickness, from 0.05 to 0.25 mm were placed inSCF for durations of three or seven days. The film formulation was 2% bymass of chitosan to solvent and 20% by mass of glycerol to chitosan in0.2 M acetic acid. It was observed that as film thickness increased, theswelling and mass loss decreased.

The films prepared from pure chitosan polymer tend to be brittle andeven crack upon drying. The addition of plasticizers to the film formingslurry may alleviate this problem. This addition of plasticizer mayimprove flexibility and possibly also the mechanical properties of thefilm. However, the addition of plasticizer may cause adverse effects onfilm properties such as increasing swelling of films in solution. Whenthe plasticizer exceeds a certain concentration, phase separation canalso occur. The amount of plasticizer used in film formation should alsobe small enough to avoid any non-biocompatible and toxic effects, yetlarge enough to increase flexibility.

Plasticizers of interest here included glycerol, ethylene glycol, poly(ethylene glycol), erythritol, oleic acid, propylene glycol, di-hydroxylstearic acid, and sorbitol. These were selected on the basis of lowcost, low toxicity, and favorable in vivo response. These are mostlypolyols which may lower the glass transition temperature for theplastic, making it more flexible at the temperature at which it will beused. This means durability should increase as a result. Of theseplasticizers, glycerol, sorbitol, and poly (ethylene glycol) are readilyavailable and low cost. Tests were done on films cast with these threeplasticizers and results are shown in FIG. 21.

Two forms of chitosan were used for this plasticizer test: medium andlow molecular weight chitosan polymers. Each of these chitosans wastested with the three plasticizers. An amount of 20% by mass of eachplasticizer to chitosan was added to the chitosan slurry for testing.The low molecular weight chitosan displayed similar results for allplasticizers in terms of swelling (tested at 7 days and 3 days), anddegradation (tested at 7 days). However, medium molecular weightchitosan was more tunable with the different plasticizers. Glycerolsamples showed less swelling compared to those with sorbitol and poly(ethylene glycol). Degradation was about the same for all plasticizersusing medium molecular weight chitosan.

From the above, glycerol plasticizer gives the chitosan films the mostfavorable characteristics and was used for further testing. Theinfluence of the amount of glycerol plasticizer in the chitosan slurrywas also tested. FIG. 22 shows the influence of concentration, by weightpercent, of glycerol to chitosan on swelling and degradation.

The greater the concentration of glycerol, the more swelling the filmsexperience in SCF. Higher glycerol concentrations also lead to greaterdegradation. There is an apparent boundary in the glycerol concentrationabove which the swelling jumps to much higher values. This boundary isbetween 20 and 30 mass percent glycerol. The data indicates that only asmall amount of glycerol is needed to produce the desired filmproperties. Accordingly, using 3%-20% by mass of glycerol to chitosanwhen forming the pH sensitive material may provide desirably lowswelling, while still maintaining the film's plasticity and flexibility.

In addition to the plasticizer, the chitosan film may includecross-linkers. The use of cross-linkers serves to provide bridges orbonds between the chains of a polymer to link them together. These bondscan be either covalent or ionic. Cross-linking agents have the abilityto improve film density and decrease the water absorption upon wettingin SCF, giving less swelling of the films as well as less degradation.Cross-linking also causes flexibility to decrease and hardness toincrease. Overall the added interactions can strengthen the chitosanfilm. The polymer chitosan can be cross-linked with such compounds assodium citrate, sodium sulfate, and calcium chloride. To cross-link, thefilms were simply allowed to soak in the cross-linking solution (acertain mass % of cross-linker mixed in water) for a fixed period oftime, after which they were rinsed with copious amounts of distilledwater to neutralize them. Sodium hydroxide was included in the slurriesas it serves to basify the films which are acidic because of the aceticacid used in the formulation; however, the sodium hydroxide does notparticipate in the cross-linking step and induces no cross-linking.Sodium citrate and sodium sulfate both provide polyvalent anions whichbind the chitosan chains to cross-link them ionically. All films werecross-linked for a period of two hours unless otherwise specified. Aftercross-linking the films were allowed to dry prior to further testing.

FIG. 23 shows the swelling and degradation results of films cast using 2mass % chitosan in the slurry and 20 mass % glycerol to chitosan in 0.2M acetic acid solvent and then treated with different cross-linkers fortwo hours with a specified mass % of cross-linker. Swelling wasdecreased when sodium citrate was used as a cross-linker compared to theothers, while the control of just NaOH and no cross-linker displayed themost swelling. The degradation was not as strongly influenced bycross-linking, as all degradation values were relatively low.

The cross-linker concentration was also tested to see its effect on thefilm swelling and degradation. For this test the sodium citrate, whichprovided low swelling and degradation, was used. A standard 7 mass % ofNaOH was used along with differing concentrations of sodium citrate.Films were cast from 2 mass % chitosan in the slurry and 20 mass %glycerol to chitosan in acetic acid and water solvent. FIG. 24 shows thefilm swelling and degradation when changing the sodium citrateconcentration from 0 to 30 mass %. Swelling at 3 days decreased as theamount of sodium citrate cross-linker was increased. The degradation didnot show a regular pattern after either 3 days or 7 days, but was nearzero at low concentrations after 7 days. A desirable sodium citrateconcentration for cross-linking was approximately 10 mass % relative tochitosan.

Another study was carried out to determine the influence cross-linkingtime on film swelling and degradation. The data from this study areshown in FIG. 25. The cross-linking time was varied from 0 (quick rinse)to 8 hours. Swelling was greatest for the low cross-linking times anddegradation was not influenced much by the cross-linking time. A shortercross-linking time is more desirable in terms of production.Accordingly, 30 minutes of cross-linking may provide favorable filmcharacteristics, namely, reduced swelling and reduced degradation whileproviding an acceptable production time.

Overall, cross-linking is a desirable step in the formation of achitosan film. The cross-linking with sodium citrate reduces swellingand minimizes degradation. Soaking for a half hour in 10 mass % sodiumcitrate relative to chitosan is sufficient to provide the desirableproperties.

Long-term degradation studies were carried out to evaluate the use ofthe additives described. The goals were to provide stability of thechitosan for months in the SCF, yet rapid degradation in the low pH ofthe fish gut. The long-term study looked at stability of the filmsimmersed in the SCF for periods of time up to 90 days. Chitosan filmswere cast with 2 mass % medium molecular weight chitosan and 20 mass %glycerol to chitosan in acetic acid and water solvent for these tests.Several cross-linkers were used for this long-term study and FIG. 26shows the results.

All of the films stood up to the SCF for the extended period of 90 days.Swelling was least in the sodium citrate cross-linked film after the 30days (101%), while the film in the SCF for 90 days and cross-linked withsodium citrate swelled approximately 112%. The degradation in the sodiumcitrate cross-linked film after the 90 days was about −2%. Overall thefilms displayed good long-term stability in the SCF.

Accelerated aging studies were done on a medium molecular weightchitosan film with glycerol cast from an acetic acid and water solution.The accelerated aging was done by placing 1×1 cm squares of the film ina 65° C. oven for 0, 2, 5, and 24 hours. Results from the acceleratedaging studies can be seen in FIG. 27.

As the films were heated to simulate aging they slightly discolored,turning from almost colorless to a yellow color and curling around theedges. This was also characteristic of films that were not subjected toaccelerated aging, i.e., films left on the bench for several months,indicating that the accelerated aging method was reliable as apseudo-aging procedure. Extreme discoloring was seen above 65° C. Thestudies also showed a film that turned brown as it is heated up totemperatures as high as 100° C.

After the films were heated to accelerate the aging process, they weretested in SCF and then an HCl acidic solution for degradation andswelling characteristics. FIG. 27 shows the swelling and degradation ofthe films after being immersed in SCF for 24 hours.

The accelerated aging tests showed that the films that were treated atelevated temperatures for extended periods of time experienced increasedswelling. This may be attributed to a breakdown of the polymerstructure, which in the case of 24 hours at 65° C. leads to extreme 300%swelling. The degradation of the accelerated aging films does not appearto be altered with the aging process, as all films still degrade as muchas others tested. It is noted that the relation between treating a filmat 65° C. for 10 hours and normal film aging is not clear; however,tests in an HCl solution to simulate a predation event show that thelonger the film was subjected to elevated temperatures, the faster itdegraded. All of the films degraded within two hours, however the filmtreated for 24 hours at 65° C. degraded in an hour. Therefore, agingdoes not appear to adversely influence the use of these films forpH-dependent degradation.

Based on the studies that were performed, a possible formulation for achitosan slurry with additives suitable for use as a pH sensitivematerial in a predation tag may be summarized as follows. A plasticizersuch as glycerol can be added in concentrations from 3% to 20% massrelative to chitosan. At this concentration the film gains flexibilitywhile avoiding extreme swelling that is characteristic of higherconcentrations of plasticizer. Cross-linking may be done with sodiumcitrate, as it decreases swelling and degradation compared to othercross-linkers. The concentration of sodium citrate that increases thestrength of the chitosan film while lowering degradation was between 5%and 30% by mass of cross-linking solution. The time for cross-linkinghad little effect on the properties of the film compared to thecross-linker concentrations, thus a half hour was sufficient.

The above formulation may be well suited for casting a film over exposedelectrodes; however, it was too runny to adhere a magnet to a tag. Themaximum concentration of chitosan in a slurry that could be stirred witha magnetic stir bar was 2 mass %. In order to increase the chitosanconcentration, the slurry had to be mixed by hand. Concentrations ashigh as 10 mass % could be achieved when mixing by hand. Slurries withchitosan concentrations from 2 to 10 mass % were tested.

The stirred slurries had trapped air bubbles and were allowed to sit anddegas overnight. The 8 and 10 mass % chitosan slurries were more like apaste. All except the 2 mass % chitosan slurry still had gas bubblesremaining after 24 hours of sitting. The 4 mass % chitosan took severaldays to degas, which may be too long for production. Above 4 mass %there were uniformity issues (clumpy), even though the chitosan did gointo solution. The 2 mass % solution degassed quickly and was uniformbut it was not viscous enough while 4 mass % was viscous enough anduniform but degassed too slowly. A slurry with 3 mass % chitosan provedacceptable for adhering a magnet to the tag. The 3 mass % slurry wasviscous enough, uniform, and degassed overnight. This 3 mass % chitosanslurry was placed drop wise on the tags with a paperclip piece (approx.5 mm in length). The paperclip piece was used to mimic the eventualsmall rare earth magnet and was situated in a small well on the tag asdepicted in FIGS. 8A-8C. The 3 mass % chitosan slurry beaded overtop thepaperclip piece and provided an effective adhesive. After the slurry wasallowed to dry it formed a thin film protecting the paperclip containedin the well.

A 300 mL batch of the 3 mass % slurry may be made with 9 g of the mediummolecular weight chitosan (3 mass %) suspended in 150 mL of distilledwater. Once suspended, 150 mL of the 0.4 M acetic acid solution mayadded. This two-step process prevents clumping of the chitosan. Thesolution may then be stirred by hand with a stirring rod. Next, 0.9 g(27 drops) of glycerol (10 mass % to chitosan) may be added drop wisewhile stirring once again with a glass stir rod. The slurry may then beallowed to sit overnight so that any bubbles that may have formed canescape (degassing).

The amount of slurry to be applied to the tags may be determinedexperimentally. Application of the slurry was found to be easiest ifdone by dripping a single drop onto a tag and then adding eachadditional drop one at a time with a drying step in between. Eight tagswith dried films were tested in SCF overnight and then degraded in anHCl solution. There were two tags each with 1, 2, 3, or 4 drops ofslurry. Multiple drops were applied by letting the prior drop dry beforeadding another. One of each of the 1, 2, 3, and 4 drop tags wascross-linked using 7% NaOH/10% sodium citrate by allowing them to soakin the cross-linking solution for two hours. The cross-linking solutionwas made by dissolving 7 g NaOH pellets and 10 g sodium citrate in 100mL distilled water. Once the two hours passed, the tags were removed andrinsed with copious amounts of distilled water until the diluent wasneutral, as tested with pH paper. The tags were then placed in SCFovernight. Tags with more than two drops showed excessive swelling inthe SCF.

After the tags sat in the SCF overnight, they were placed in weaklyacidic HCl solution (pH˜3) to test the time for degradation. Theadhesive was determined to be degraded when the adhered paperclip piecefell from its well. The tags were checked every half hour by shaking thevial they were contained in. Table 2 shows the time it took fordegradation of the adhesive according to the number of drops applied andwhether it was cross-linked.

TABLE 2 Hours to degrade chitosan adhesive on different tags. Number ofdrops of slurry Cross-link Time to degrade (hours) 1 No 2.5 1 Yes 3 2 No5 2 Yes 6 3 No 7 3 Yes 8 4 No 11 4 Yes 12

The adhesive always degraded on the order of hours. As expected, themore drops of slurry on the tag, the longer degradation took. Theresults also show that the cross-linking step usually added an extrahour to the degradation time. It is desirable that the chitosan degraderelatively rapidly in the low pH of the fish gut. Digestion can takefrom a couple of hours to a day or so in fish. One or two drops ofmedium molecular weight chitosan slurry with a cross-linking step wouldbe sufficient in order to keep the degradation time around six hours orless. It should be noted that in a fish gut the tags would be subjectedto harsher treatment in terms of churning action and a lower pH.

Several small tags using rare earth magnets measuring approximately 1mm×0.5 mm were tested. In the tag tested, a battery was in closeproximity to where the magnet was adhered. As a result, the magnet wasattracted to the metal components of the battery, making adhesiondifficult. The magnet had to be held in place while the chitosanadhesive dried in order for the magnet to remain in the correct positionand not jump to the battery. For this to occur, only a small drop ofslurry was initially applied beneath the magnet while a plastic piecenot touching the adhesive, such as tweezers, held the magnet. Thechitosan adhesive dried to give an initial bond between the magnet andtag sufficient to prevent the magnet from jumping to the metalcomponents of the battery. After drying was complete, the plastic piececould be removed and a second drop of chitosan adhesive could be appliedto coat the rest of the magnet and provide protection and robustness tothe magnet and tag once implanted. The second drop could be from thesame slurry or a different one, meaning the concentration of chitosan inthe two drops can be different, such as 2, 3 or 4 mass %, as it hasrelatively little effect on the properties of the final product.

Tags with the small rare earth magnets adhered as described were testedin SCF. The devices were tested for a period of seven days of immersionin SCF and the magnet remained in place during this time period,although some swelling of the adhesive was apparent to the naked eye.After being in the SCF, the tags were transferred to an HCl solutionthat simulated a predation event. The adhesive quickly degraded, withina half hour, and the magnet fell from its place, usually jumping to thebattery close-by. This degradation was much quicker than the regulartags that were tested using a paperclip which may be attributed to theattractive force between the rare earth magnet and the magneticcomponents of the battery nearby. The paperclip, in contrast, would fallout of its place due to gravity.

The chitosan slurry described above may shrink during the dryingprocess. Depending upon how the chitosan slurry is used, for example asan adhesive, the shrinking may not present a challenge. However, inother uses, such as for forming a plug or filling a cavity within thetag body, such shrinking may be undesirable or problematic.

The shrinkage may be controlled or mitigated by the addition of a solidfiller material. The solid filler material may be microspheres althoughother filler material, alone or in combination may be used. In testingof the shrinkage of the chitosan slurry with solid filler material, 3M™brand K37 glass microspheres were used. K37 glass microspheres are acommercially available powdered filler that is lightweight andchemically non-reactive. The filler occupies most of the space, whilethe chitosan acts as a pH sensitive binder to hold the filler particlestogether until the tag enters the acidic environment of the predator'sstomach. At this point the chitosan degrades allowing the compositematerial to disintegrate.

The dried chitosan/microspheres composite can be relatively easilycrushed. This may be beneficially exploited in the magnetic tagsdescribed above with reference to FIGS. 9, 10 and 11, since a depressioncan easily be formed in the dried material by pressing a tool into thesurface. The shape of the depression may correspond to the shape of amagnet. The magnet can be inserted into the depression in order tosecure the magnet in place. The depression within the pH sensitivematerial may provide a degradable physical barrier that prevents or atleast resists movement of the magnet while the pH sensitive materialremains intact or non-degraded.

A range of concentrations of filler were tested to determine theresulting shrinkage rate and other effects on the material properties.In table 3 below the “Volume % Filler” is the volume of 3M™ brand K37glass microspheres as a percentage of the volume of the chitosan slurryafter diluting the slurry 1:1 with water to facilitate processing.“Volume % Shrinkage” is the percentage reduction in volume after drying.

TABLE 3 Table showing Volume % shrinkage of Chitosan slurry with varyingamounts of filler Volume % Filler Volume % Shrinkage 27.7% 56% 55.5% 18%83.2% Approximately 0%  111% Approximately 0%  139% Approximately 0% 166% Approximately 0%

The more filler that was added, the more viscous the wet mixture became.The addition of 83.2-111% glass microspheres substantially eliminatedshrinkage upon drying and resulted in a wet mixture with a viscositythat was easily dispensed. The rate of degradation of the driedcomposite material in an acid environment was not substantiallyaffected, nor was its survival in a neutral or basic environment. Withthe addition of 166% microspheres the mixture was so viscous it wasdifficult to dispense, and was brittle and crumbly when dry.

Various predation tags have been described above that utilize a pHsensitive material in order to provide a measurable change when the tagis in an acidic environment. A chitosan slurry formulation has beendescribed, which may be useful as the pH sensitive material. The slurrymay be used in adhering rare earth magnets on fish tags as a way todetect predation events, or for coating electrodes or other sensors.

Although specific embodiments of predation tags have been described,along with various formulations for pH sensitive material that is suitedfor use with the predation tag, other embodiments may be provided thatchange a measurable characteristic when in the presence of an acidicenvironment. The change in the characteristic can be detected by the tagand used to adjust the operation of the tag in order to indicate that apredation event was detected. It will be appreciated by those skilled inthe art that other forms, arrangements or configurations of tags,sensors and related materials may be employed to provide an indicationas to whether an animal being tracked was eaten by a predator.

What is claimed is:
 1. A tag for tracking an animal comprising: a firstmagnet at least partially encased in a pH sensitive material exposed toan external environment of the tag, the pH sensitive material degradingin the presence of an acidic environment; a sensor for detecting amagnetic field; a magnetic attractor providing a force to induce achange in the magnetic field detectable by the sensor upon release ofthe first magnet as a result of at least partial degradation of the pHsensitive material; and circuitry for providing a predation eventtrigger based upon the change in the magnetic field detectable by thesensor.
 2. The tag of claim 1, wherein the magnetic attractor comprisesa second magnet.
 3. The tag of claim 2, wherein the second magnet ispermanently affixed to the tag.
 4. The tag of claim 3, wherein thesecond magnet causes the first magnet to move relative to the sensorwhen the pH sensitive material is sufficiently degraded to release thefirst magnet.
 5. The tag of claim 4, wherein the second magnet isarranged with its magnetic field perpendicular to the magnetic field ofthe first magnet when the first magnet is at least partially encased ina pH sensitive material, and the second magnet causes the first magnetto rotate when the pH sensitive material is sufficiently degraded torelease the first magnet.
 6. The tag of claim 5, wherein the sensor iscapable of detecting an orientation of a magnetic field, a change in theorientation of the magnetic field, a magnitude of the magnetic field, achange in the magnitude of the magnetic field or a combination thereof.7. The tag of claim 6, wherein the second magnet is arranged with itsmagnetic field perpendicular to a particular axis of the sensor.
 8. Thetag of claim 7, wherein the sensor is sensitive to the magnetic field ofthe first magnet when it is oriented parallel with the particular axisof the sensor, and insensitive to the magnetic field of the first magnetthat is oriented perpendicular to the particular axis of the sensor. 9.The tag of claim 2, wherein the second magnet attracts the first magnetout of a detection range of the sensor when the pH sensitive material issufficiently degraded to release the first magnet.
 10. The tag of claim2, wherein the first magnet and the second magnet are secured or locatedagainst or adjacent to each other by the pH sensitive material withtheir respective magnetic fields aligned such that their respectivemagnetic fields add together constructively, wherein the first magnetand the second magnet tend to rotate or move relative to each other whenreleased from the pH sensitive material such that their respectivemagnetic fields add destructively when moved or rotated.
 11. The tag ofclaim 9, wherein the sensor is capable of detecting a strength of themagnetic fields or a change in the strength of the magnetic fields. 12.The tag of claim 1, wherein the magnetic attractor comprises anun-magnetized ferromagnetic material permanently affixed to the tag. 13.The tag of claim 12, wherein the magnetic attractor attracts the firstmagnet out of a detection range of the sensor when the pH sensitivematerial is sufficiently degraded to release the first magnet.
 14. Thetag of claim 1, wherein the first magnet comprises a degradable magnet.15. The tag of claim 14, wherein the degradable magnet comprisesparticles of ferromagnetic material bonded together by the pH sensitivematerial, the particles of the ferromagnetic material being magnetizedsubsequent to bonding together to impart a net magnetic field to thedegradable magnet.
 16. The tag of claim 15, wherein the degradablemagnet releases the magnetized particles of magnetic material when thepH sensitive material is degraded resulting in a reduced net magneticfield.
 17. The tag of claim 1, wherein the acidic environment is a gutof a predator animal.
 18. The tag of claim 1, wherein the pH sensitivematerial does not degrade substantially in a neutral or basicenvironment.
 19. The tag of claim 18, wherein the neutral or basicenvironment is a coelomic cavity of the animal.
 20. The tag of claim 1,wherein the first magnet is affixed to the tag by using the pH sensitivematerial as an adhesive to affix the first magnet to the tag.
 21. Thetag of claim 1, wherein the first magnet is at least partially encasedin the pH sensitive material to form a plug that is affixed to the tagusing an adhesive.
 22. The tag of claim 1, wherein the first magnet isat least partially encased in the pH sensitive material to form a plugthat is mechanically retained by at least a portion of a body of thetag.
 23. The tag of claim 1, wherein the pH sensitive material comprisesa chitosan.
 24. The tag of claim 23, wherein the pH sensitive materialis cast from a slurry of the chitosan and a solvent.
 25. The tag ofclaim 24, wherein the solvent is selected from: L-ascorbic acid; citricacid; acetic acid; and hydrochloric acid.
 26. The tag of claim 24,wherein the solvent is citric acid.
 27. The tag of claim 24, wherein thesolvent is acetic acid.
 28. The tag of claim 23, wherein the pHsensitive material comprises a film having a thickness of at least 0.05mm.
 29. The tag of claim 23, wherein the pH sensitive material comprisesa film having a thickness of at least 0.20 mm.
 30. The tag of claim 23,wherein the pH sensitive material comprises a plasticizing agent. 31.The tag of claim 30, wherein the plasticizing agent is selected from:glycerol; ethylene glycol; poly ethylene glycol; erythritol; oleic acid;propylene glycol; di-hydroxyl stearic acid; and sorbitol.
 32. The tag ofclaim 31, wherein the plasticizing agent is glycerol.
 33. The tag ofclaim 23, wherein the pH sensitive material is treated with across-linking agent.
 34. The tag of claim 33, wherein the cross-linkingagent is selected from: sodium citrate; sodium sulfate; and calciumchloride.
 35. The tag of claim 23, wherein the pH sensitive materialcomprises a filler material to control shrinkage of the pH sensitivematerial.
 36. The tag of claim 35, wherein the filler material comprisesmicrospheres.
 37. The tag of claim 36, wherein the microspheres compriseglass microspheres.
 38. The tag of claim 35, wherein the filler materialis present in an amount of between 50% and 150% by volume of the pHsensitive material prior to drying.
 39. The tag of claim 38, wherein theamount of filler material present is between 83% and 111% by volume ofthe pH sensitive material prior to drying.
 40. The tag of claim 1,further comprising a microprocessor for controlling one or morefunctions of the tag based upon the predation event trigger.
 41. The tagof claim 40, wherein the circuitry for providing the predation eventtrigger is provided by the microprocessor.
 42. The tag of claim 40,wherein the microprocessor operates in at least one of a first mode or asecond mode based on the predation event trigger.
 43. The tag of claim42, wherein the microprocessor switches from operating in the first modeto operating in the second mode based on the predation event trigger.44. The tag of claim 40, wherein the microprocessor logs informationrelated to the predation event trigger.
 45. The tag of claim 44, whereinthe microprocessor logs information to non-volatile memory of the tag.46. The tag of claim 44, wherein the microprocessor maintains a measureof elapsed time since the predation event trigger, and wherein thelogged information comprises the elapsed time since the predation eventtrigger event.
 47. The tag of claim 40, further comprising a transmitterfor transmitting information related to the predation event trigger. 48.The tag of claim 47, wherein the transmitter comprises an acoustictransducer.
 49. The tag of claim 47, wherein the transmitter comprises aradio frequency (RF) transmitter.
 50. The tag of claim 49, wherein theRF transmitter comprises an active RF transmitter.
 51. The tag of claim49, wherein the RF transmitter comprises a passive RF transmitter. 52.The tag of claim 51, wherein the passive RF transmitter comprises anRFID transmitter.
 53. The tag of claim 47, wherein the microprocessormaintains a measure of elapsed time since the predation event trigger,and wherein the transmitted information comprises the elapsed time sincethe predation event trigger event.
 54. The tag of claim 53, wherein themeasure of elapsed time transmitted by the transmitter is encoded innon-linear fashion.
 55. The tag of claim 40, wherein the microprocessoroperates in at least a configuration mode for transferring data to thetag to configure operation of the microprocessor.
 56. The tag of claim55, wherein a varying magnetic field is used for transferring data tothe tag when the microprocessor is in the configuration mode.
 57. Thetag of claim 56, wherein the microprocessor further operates in at leasta calibration mode for determining a value for a compensation magneticfield to allow detecting of the varying magnetic field used fortransferring data in the presence of a constant magnetic field of atleast the first magnet.
 58. The tag of claim 57, wherein themicroprocessor calculates the value for the compensation magnetic fieldand transmits the calculated value for the compensation magnetic fieldto an activation device.
 59. The tag of claim 57, wherein the tagtransmits an indication of a detected magnetic field in order to allowan activation device to calculate the value for the compensationmagnetic field.
 60. The tag of claim 1, wherein the animal is an aquaticanimal.